In data processing systems which utilize a control store for storing microcode (i.e., microinstructions) for use in implementing programs in response to the decoding of macroinstructions, for example, the microcode which is loaded therein is usually fixed in nature and only a single set of microcode instructions can be normally so loaded. Such conventional systems, accordingly, tend to be less flexible since they are limited to the loading of only a single set of microinstructions. Moreover, a special controller is normally required in order to access the microcode from an appropriate I/O device (such as a floppy disk, for example) on which the microcode is stored and such controller is normally designed to access only the I/O device which contains the fixed set of microcode instructions.
It is desirable, therefore, in order to provide more flexibility to such a microcode system to be able to access more than one set of microinstructions from more than one I/O device and to do so without the need for one or more special controllers for such purpose associated with each said I/O device.